Sonny Bill Williams has admitted that his return to rugby league with the Sydney Roosters was a 'pretty tough' decision to make.

On Tuesday Williams announced his new one-year contract with the Roosters, which marks his return to the sport after four years' absence.

The contract also represents Williams' commitment to a handshake deal he made with Roosters chairman Nick Politis three years ago.

The pressure is on me as much as it is on the club. I've been out of the game for five years, so it's a big challenge. All I can do is stay humble, work hard and try my best.

Sonny Bill Williams

Since 2008 the former Canterbury Bulldogs player has played rugby union in France and New Zealand - including 19 caps for the All Blacks.

However he announced earlier this year he would move to Japan for a lucrative short-term stint with rugby union side the Panasonic Wild Knights before moving to an un-named NRL club in 2013.

Williams confirmed on Tuesday that the Rooster would be that team, but said he has grown to love rugby union and admitted it was difficult to leave the code when honouring his deal with Politis.

"When I did the deal, a lot of great things happened to me in that space, especially going back to New Zealand, winning the Super Rugby with the Chiefs, the World Cup with the All Blacks, a lot of great things have happened," he said.

"If I'm honest with myself I must admit it was pretty tough to honour what me and Nick agreed about.

"It's going to be a massive challenge. I've met the boys, they're a great bunch of blokes, real young but real enthusiastic and welcoming too.

"I'm looking forward to getting back out there in the game I grew up playing."

The controversial 27-year-old's stint with the Wild Knights hit a snag when he sustained a pectoral injury during a game in late October which required chest surgery.

The surgery was successful, but it could keep Williams out of action for two to four months.

"I've signed a one-year-deal with the Roosters. I've been out of the game for a long time but it's terrific to be back," Williams said.

"I'd like to thank (Roosters chairman) Nick Politis who approached me three years ago with the deal. At the moment I'm still a rugby player, they've given me permission due to the [pectoral] injury."

Despite the deal only lasting one year, Williams expects it to be renewed, all things going well.

"For me, the last four years I've just signed one-year deals," he said.

"I consider myself a boxer as well, now. I think the two for me go hand in hand. It brings the best out of me.

"The pressure is on me as much as it is on the club. I've been out of the game for five years, so it's a big challenge. All I can do is stay humble, work hard and try my best."

Code comeback

Roosters coach Trent Robinson says he has watched Williams play in Japan and, after a number of meetings in getting the deal done, is convinced the player is serious about his NRL return.

"I was pretty clear after [meeting Williams in Japan] that he was serious about a return but also that process of becoming a professional athlete both in boxing and rugby league," he said.

The decision means Williams will again play in 2013 the sport where he made his name.

In 2004 he made his first-grade debut for the Bulldogs at 18, and he made his international debut the same year when selected for New Zealand in the Anzac Day Test match against Australia.

He played in a Premiership side in his first season as the Bulldogs defeated the Roosters 16-13 in the Grand Final.

In 2007 he re-signed with the Bulldogs for five years, but he caused a storm of controversy when he walked out on that contract just 18 months later, switching codes to play rugby union for French Top 14 side Toulon.

He played in France for three years before opting to take a sizeable pay cut to play Super Rugby in New Zealand for the Crusaders - and later for the Chiefs - citing a desire to play for the All Blacks.

The move paid off, with Williams playing in 19 Tests for New Zealand, including a number of appearances in the All Blacks' Rugby World Cup triumph in 2011.

He continued to play in internationals in 2012, making a big impact in New Zealand's home series against Ireland, and scoring against the Wallabies in the first Bledisloe Cup match in Sydney in August.

His contract with the New Zealand Rugby Union expired after he was part of the Chiefs' first Super Rugby title win.

A hard-running, hard-hitting centre or second-rower, Williams will provide attacking options for the Roosters and new coach Robinson.

Williams has also made headlines with a heavyweight boxing career.

He was due to fight South African boxer Francois Botha in Brisbane on November 24, but that bout was put in doubt after succumbing to his pectoral injury.